Polyphenylene ethers not only have excellent mechanical and electrical properties and excellent heat resistance, but also have excellent dimensional stability. Therefore, polyphenylene ethers have been used in a wide variety of fields. However, the moldability of polyphenylene ether is poor. Nowadays, polyamide-polyphenylene ether alloys, in which this disadvantage, moldability, has been improved, are used in a wide variety of fields.
Recently, use of a conductivity-imparted polyamide-polyphenylene ether alloy is rapidly expanding in the fields of automobile outer panels (such as a fender and a door panel) as an electrostatically coatable material. For example, it has been attempted to use an automobile fender made of a polyamide-polyphenylene ether alloy for the purpose of improving not only the safety of automobiles (e.g., protection of pedestrians) but also the ability of an automobile fender to recover from distortion.
Generally, in order to produce a conductive resin, a resin is mixed with a carbonaceous filler such as carbon black. For example, with respect to a technique for imparting conductivity to a polyamide-polyphenylene ether alloy, there is disclosed a conductive masterbatch comprising conductive carbon black present in the form of at least one agglomerated particle having a major axis of 20 to 100 μm, wherein the number of the agglomerated particle(s) is 1 to 100 (Patent Document 1). Further, there is disclosed a technique to obtain a conductive resin mixture by uniformly dispersing carbon black in a polyamide in advance, followed by mixing thereof with a polyphenylene ether (Patent Document 2). There is disclosed a technique in which a polyamide and a polyphenylene ether are compatibilized with each other in advance, followed by addition of carbon black, to thereby obtain a resin composition having good impact strength, good melt-fluidity, and good (low) volume resistivity (Patent Document 3).
There is disclosed a method of obtaining a conductive masterbatch by a method of adding a solid resin and carbon fibril to an extruder or the like simultaneously, followed by mixing thereof (Patent Document 4). Further, there is disclosed a method of adding carbon black to a polyamide resin that is melted in advance, followed by mixing thereof (Patent Document 5). There is disclosed a method of obtaining a conductive masterbatch by mixing a first polyamide powder with carbon black in advance and then compounding this polyamide powder/carbon black mixture with a second resin (Patent Document 6).
However, these techniques are all lab scale techniques using small extruders, and they do not take into consideration of the production of a conductive masterbatch in large quantities stably for a long time by means of a large extruder. Generally, when a conductive masterbatch is produced for a long time using a large extruder, decomposition of resin is promoted by an increase in resin temperature and die drips (a lump of resin which grows at the die exit of an extruder with time) are formed in large quantities. A formation of the die drips poses problems. For example, the die drips cause strand breakage when a certain size is reached, or the die drips are carried with a strand and mixed into a product pellet. Furthermore, the strand breakage by the poor distribution of a carbonaceous filler also occurs.
As described above, it was difficult to produce a conductive masterbatch stably for a long time using a large extruder, and the die drips and the poor distribution of a carbonaceous filler were major factors to reduce productivity.    [Patent Document 1] WO 04/60980    [Patent Document 2] JP-A-2-201811    [Patent Document 3] JP-A-8-48869    [Patent Document 4] National Publication of International Patent Application No. 8-508534    [Patent Document 5] JP-A-2004-2898    [Patent Document 6] U.S. Pat. No. 2004-0238793